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  • 1
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    In:  Geological Society Special Publication 353: 109-125.
    Publication Date: 2011-03-10
    Description: A seismic velocity cross-section down to 700 km depth beneath the Tibetan Plateau has been constructed. Beneath the cover layer, felsic rocks rich in a quartz exist down to 15–25 km depth. Beneath these depths, temperatures are probably high enough for ductile flow and partial melting to occur. The velocity increase across the boundary at 30–40 km depth marks the interface between felsic upper crust and more mafic lower crust. Crustal thickness is greatest (c. 74 km) south of c. 31.5°N, where Indian lower crust forms the basal layer. Northwards, crustal thickness decreases to c. 66 km around 33°N, before increasing to c. 70 km beneath northern Tibet. Crossing the Kunlun, the crust thins to c. 54 km beneath the Qaidam basin. High-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong-Nujiang suture, where it downwells to 350–400 km depth. The lithosphere–asthenosphere boundary occurs at 160–225 km depth. The apparent northwards deepening of the 410 and 660 km discontinuities implies that the upper mantle beneath northern Tibet is slower, less dense and warmer than under southern Tibet which, in turn, could provide some of the isostatic support for the high elevations in northern Tibet where the crust is thinner than under southern Tibet.
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  • 2
    Publication Date: 1996
    Keywords: URSEIS 95, Urals, tectonics/crustal structure, refraction seismics
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  • 3
    Publication Date: 1999
    Keywords: refraction seismics, URSEIS 95, Urals, tectonics/crustal structure
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  • 4
    Publication Date: 2019-07-16
    Description: The European Plate has a 4.5 Gy long and complex tectonic history. This is reflected in the present-day large-scale crustal structures. A new digital Moho depth map is compiled from more than 250 data sets of individual seismic profiles, 3-D models obtained by body and surface waves, receiver function results and maps of seismic and/or gravity data compilations. We have compiled the first digital, high-resolution map of the Moho depth for the whole European Plate, extending from the mid-Atlantic ridge in the west to the Ural Mountains in the east, and from the Mediterranean Sea in the south to the Barents Sea and Spitsbergen in the Arctic in the north. In general, three large domains within the European Plate crust are visible. The oldest Archean and Proterozoic crust has a thickness of 40–60 km, the continental Variscan and Alpine crust has a thickness of 20–40 km, and the youngest oceanic Atlantic crust has a thickness of 10–20 km.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the Pn phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the Pn phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P-wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s−1 at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed Pn amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s−1 in the fast direction to 0 s−1 in the slow direction of horizontal P-wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In August 1985 the crustal structure underlying the southern part of the Kenya Rift Valley was investigated by long-range explosion seismology. the experiment (KRISP 85) consisted of two seismic lines in the central sector of the rift, one along the axis and the other across it. Interpretation of the data, including time-term analysis and ray tracing has shown that the thickness of rift infill varies from about 6km below Lake Naivasha to about 2 and 1.5km below Lake Magadi and Lake Bogoria respectively. the underlying material has a P-wave velocity of 6.05 ± 0.03 km s-1 which suggests that the rift is underlain by Precambrian metamorphic basement. A localized high-velocity zone identified to the east of Nakuru may be due to basic intrusive material. the P-wave velocity increases discontinuously to 6.45 ± 0.2 km s-1 at a depth of 12.5 ± 1.0 km below sea level. This depth is similar to that inferred for the brittle-ductile transition zone from a study of local seismicity in the Lake Bogoria region. A high P-wave velocity layer (7.1 ± 0.2 km s-1) occurs at 22 ± 2 km depth below sea level which might be associated with a sill-like basic intrusion in the lower crust. an upper mantle velocity of 7.5 ± 0.2 km s-1 (unreversed) is reached at a depth of 34.0 ± 2.0 km below sea level. This implies that only moderate crustal thinning has occurred beneath the central sector of the rift. No evidence was obtained for the existence of a continuous‘axial intrusion’reaching to shallow levels below the rift and associated with crustal separation as suggested by previous studies.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A unified two-dimensional lithospheric model has been derived for the P-wave velocity structure beneath the Alpine Longitudinal Profile (ALP'75) main line by making new phase correlations and using ray-tracing and ray-theoretical seismogram calculations in laterally inhomogeneous media. Due essentially to an increase in thickness of the lower crust, the total crustal thickness increases from 30–40 km in the peripheral regions to about 50 km in the central region of the Alps. The thickness change is more abrupt at the eastern end than at the western end of the line. The boundary between the upper and lower crust occurs at 18–20 km depth except in the peripheral region at the eastern end of the line where it occurs at 14–16 km depth. Along the whole line the seismic basement has velocities averaged over some tens of kilometres of 5.85-6.15 km s-1 with individual values having uncertainties of ±0.05-0.1 km s-1 at least east of shot-point B. The lower part of the upper crust has been modelled as a low velocity layer, which is most pronounced beneath the peripheral regions, and which is generally underlain by a transition zone at the base of which the top of the lower crust occurs. The top of the lower crust has been modelled with velocities ranging from 6.25-6.5 km s-1 beneath the peripheral regions to 6.5-6.65 km s-1 beneath the central Alps. Velocity uncertainties at the top of the lower crust are estimated to be around ±0.2 km s-1. Beneath the central Alps a low velocity layer has also been modelled in the lower crust. The top 6 km of the mantle has been modelled with velocities of 8.0–8.2 km s-1 with individual estimates having uncertainties of ±0.15 km s-1. Below this, the upper mantle down to almost 100 km depth has been modelled as a series of high velocity layers embedded in material with lower velocities of 8.0–8.2 km s-1. The combined effects of tectonic overthrusting and uplift and erosion have probably led to the lower crust being apparently thicker than the upper crust in the central Alps at the present day.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Records of densely spaced shots along the Sino-US reflection line INDEPTH II at offsets between 70 and 130 km parallel to the main profile provide an image of the crust straddling the Indus-Yarlung suture. The major features are prominent reflections at about 20 km depth beneath and extending out to about 20–30 km north and south of the surface exposure of the suture, and north-dipping reflectors north of the suture. Various interpretations for the reflections are possible. (i) They represent a decollement, possibly of the Gangdise thrust system. In this scenario, the surface expression of the Gangdise thrust as mapped in eastern south Tibet is a splay with the decollement continuing southwards and either ending as a blind thrust or ramping up as one of the thrusts within the northernmost Tethyan shelf sequence. (ii) The reflections represent fabrics within gneisses, partly obliterated by intrusions reaching various levels of the crust. The reflection bands may be interpreted in terms of deformation or sedimentary structures belonging to the Indian crust, the accretionary complex, and the basement of the Gangdise belt. The intrusions could be related to the Tethyan leucogranites south of the suture (Rinbung leucogranite), and to the Gangdise magmatic arc to the north of the suture. (iii) The reflections represent a fortuitous coincidence of different features north and south of the suture. South of the suture, the reflections may record the basement–cover interface of the Indian crust or a thrust system in the Tethyan shelf. North of the suture, they may comprise different levels within the Gangdise belt and its basement. Although it is not possible to discriminate between the suggested scenarios without additional information, the seismic mapping points to the importance of post-collisional (Oligocene–Miocene) tectonics, which reshaped the suture.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: During the Kenya Rift International Seismic Project (KRISP 90) a 450 km long E-W seismic-refraction/wide-angle-reflection profile involving the deployment of 250 instruments was shot across the Kenya Rift. A reflected phase recorded between distances of 260 and 350 km from a 1000 kg shot at the western end of the line in Lake Victoria has been interpreted as originating from about 60 km beneath the western margin of the rift.Detailed processing of this phase has resulted in defining its polarity in relation to the first-arrival diving wave at the same range. Extensive kinematic and dynamic modelling shows there is a high-velocity zone at depths below 60 km under the western flank of the rift. We cannot exclude the presence of a layered alternating high-low-velocity structure as found in the upper mantle beneath the northern part of the N-S seismic profile along the rift axis.Constraints from xenolith studies indicate that anisotropy may explain the high velocity found beneath the reflecting horizon (≥8.40km s−1). Petrological modelling shows that if the anisotropy is due to the preferred orientation of olivine crystals, then either a transverse isotropic structure, in which the ‘a’ and ‘c’ axes are randomly orientated in the horizontal plane, or an orthorhombic structure, in which the fast ‘a’ axis is orientated along the direction of the E-W seismic line, is possible. The reflection could also be caused by a pre-rift structure associated with the Proterozoic collisional orogen involving the Mozambique Orogenic Belt and the Archaean Nyanza Craton, whose contact is subparallel to and lies about 70 km to the west of the Tertiary rift. The evidence presented here delimits the lateral extent of the upper-mantle region of anomalously low-velocity material that is confined to below the surface expression of the rift itself at depths below 60 km.
    Type of Medium: Electronic Resource
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